1. Field of the Invention
The present invention relates to an X-ray imaging apparatus configured to obtain an X-ray phase contrast image by means of Talbot interferometry.
2. Description of the Related Art
X-ray Talbot interferometry is known as an X-ray phase contrast imaging using phase differences of X-rays.
Talbot interferometry is a technique of reconstructing a phase image of an object from an interference pattern that appears under certain interference conditions. At least an X-ray source configured to emit spatially coherent X-rays, a grating configured to diffract the X-rays and an X-ray detector configured to detect the interference pattern (self-image) that is formed as the grating diffracts the X-rays are required to form an image of an object by means of Talbot interferometry. As an object is placed between the X-ray source and a phase grating and X-rays are irradiated onto the object, a phase shift takes place while the irradiated X-rays are transmitted through the object. The self-image that is formed while the X-rays transmitted through the object are diffracted by the grating provides phase information on the object so that a phase image of the object can be obtained by detecting and analyzing the self-image. However, since a self-image formed as a result of interference of X-rays has a very small period, X-ray detectors that are popularly being employed for imaging apparatus more often than not show an insufficient spatial resolution. Therefore, a technique of placing an amplitude grating (absorption grating), which is a grating formed by arranging X-ray transmitting portions and X-ray shielding portions, at the position (Talbot position) where the X-rays diffracted by a grating form a self-image to produce Moire fringes and detecting the produced Moire fringes is being popularly employed. Since information on the phase shift of X-rays produced by an object can be detected by way of deformed Moire fringes, an image of the object can be formed by detecting the Moire fringes by means of an X-ray detector.
When an image of an object is formed in a room such as an ordinary laboratory room, generally an X-ray tube is employed as X-ray source and an X-ray detector is placed at a position separated by only a few or several meters from the X-ray source. Therefore, X-rays are divergent X-ray beams emitted from a small source. In such a case, the parallelism between the X-ray shielding portions and the X-rays entering the amplitude grating is lost to a large extent in a peripheral area of the amplitude grating.
Then, as a result, the ratio of the X-rays that enter obliquely to the X-ray shielding portions of the amplitude grating rises particularly in the peripheral area of the amplitude grating and the contrast of intensity distribution falls in a peripheral area of the X-ray detector. To cope with this problem, Japanese Patent Application Laid-Open No. 2007-206075 (corresponding to U.S. Pat. No. 7,486,770) proposes a technique of using a curved amplitude grating or tilting the X-ray shielding portions of an amplitude grating toward an X-ray source.